I think you're missing the point.
Of course what he's said is effectively true, but somebody in his position should not be saying it to the press.
The organisation that I work for can sometimes be less effective than I would like. I can say that to my boss, and he will agree. We can go for a pint and chat about how frustrating we find it, and there's absolutely no issue. If I write something on social media criticising the organisation however, I would expect to be disciplined for it, and rightly so. How can the public have faith in us if staff are openly criticising policy? They can't, and the same applies here. Moshiri is trying to sell the idea that we're going places, and Koeman is saying we aren't - it's ridiculous. All he needed to say was 'We really want Lukaku to stay and we feel like in time he can achieve everything he wants here, but we appreciate that he wants to win things and play at the top level so if we can't offer him that it might be difficult to hold on to him.' It's not rocket science.